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© 2014 Pearson Education, Inc.

Plate Tectonics and

the Ocean Floor

Chapter 1 Clickers

Essentials of OceanographyEleventh Edition

Alan P. Trujillo

Harold V. Thurman


Chapter 2 Lecture


Chapter Overview

• Much evidence supports plate tectonics

theory.

• The plate tectonics model describes features

and processes on Earth.

• Plate tectonic science has applications to

Earth Science studies.

• Configuration of land and oceans has

changed in the past and will continue to

change into the future.

© 2014 Pearson Education, Inc.© 2014 Pearson Education, Inc.

Plate Tectonics

• Alfred Wegener

first proposed in

1912

• Called it

“Continental Drift”


Evidence for Continental Drift

• Wegener proposed

Pangaea – one large

continent existed

200 million years ago

• Panthalassa – one

large ocean

– Included the Tethys

Sea

• Noted puzzle-like fit of

modern continents



• Puzzle-like fit

corroborated in 1960s

• Sir Edward Bullard

used computer

models to fit

continents.



• Matching sequences

of rocks and mountain

chains

• Similar rock types,

ages, and structures

on different continents



• Glacial ages and other

climate evidence

• Evidence of glaciation

in now tropical regions

• Direction of glacial flow

and rock scouring

• Plant and animal fossils

indicate different

climate than today.



• Distribution of

organisms

• Same fossils found on

continents that today

are widely separated

• Modern organisms

with similar ancestries


Objections to Early Continental Drift Model

• 1915 – Wegener published The Origins of

Continents and Oceans

– Suggested continents plow through ocean

basins

• Met with hostile criticism and open ridicule

• Tidal gravitational attractions too small to

move continents

• Proposed mechanism defies laws of physics


Evidence for Plate Tectonics

• New evidence from World War II

• Sea floor studies with sonar

• New technology enabled study of Earth’s

magnetic field



• Earth’s magnetic field

and paleomagnetism

• Earth has magnetic

polarity

• North and South

polarities

• Magnetic polarity

recorded in igneous

rocks

– Magnetite in basalt



• Paleomagnetism –

study of Earth’s

ancient magnetic field

– Interprets where rocks

first formed

– Magnetic dip


Earth’s Magnetic Pole



• Apparent polar

wandering

• Location of North

Pole changed over

time

• Magnetic dip data


Magnetic Polarity Reversals

• Earth’s magnetic

polarity reverses

periodically

• Recorded in ancient

igneous rocks

• 176 reversals in past

76 million years

• Unpredictable pattern


Paleomagnetism and the

Ocean Floor

• 1955 – deep water rock mapping

• Magnetic anomalies – regular pattern of

north-south magnetism “stripes”

• Stripes were symmetrical about long

underwater mountain range


Sea Floor Spreading

• Harry Hess

– World War II submarine captain and geologist

• Depth recordings show sea floor features

• History of Ocean Basins

– Seafloor spreading

– Mantle convection cells as driving mechanism


Plate Tectonic Processes


Sea Floor Spreading

• Mid-ocean ridge – spreading center

• Subduction zones – oceanic trench site of

crust destruction

• Subduction can generate deep ocean

trenches.


Sea Floor Spreading Evidence

• Frederick Vine and

Drummond Matthews

(1963)

• Analysis of igneous

rock stripes around

mid-ocean ridge

• Sea floor stripes

record Earth’s

magnetic polarity


Age of Ocean Floor

• Late 1960s deep-sea drilling

• Radiometric dating of ocean rocks

• Symmetric pattern of age distribution about

mid-ocean ridges

• Oldest ocean floor only 180 million

years old


Age of Ocean Floor


Heat Flow

• Heat flow – heat from Earth’s interior

released to surface

• Very high at mid-ocean ridges

• Low at subduction zones


Earthquakes as Evidence

• Most large earthquakes occur at subduction

zones.

• Earthquake activity mirrors tectonic plate

boundaries.


Global Plate Boundaries


Plate Tectonics Theory

• Lithosphere – tectonic plates that float on ductile

asthenosphere

• Large-scale geologic features occur at plate

boundaries.

• Two major tectonic forces

– Slab pull

– Slab suction


Types of Plate Boundaries


Examples of Plate Boundaries


Divergent Boundary Features

• Plates move apart

• Mid-ocean ridge

– Rift valley

• New ocean floor

created

• Shallow focus

earthquakes

– Intensity measured

with seismic moment

magnitude


Divergent Plate Boundary


Generation of a Divergent Boundary


Formation of a Rift Valley


Types of Spreading Centers

• Oceanic rise– Fast-spreading

– Gentle slopes

– East Pacific

• Oceanic ridge– Slow-spreading

– Steep slopes

– Mid-Atlantic

• Ultra-slow– Deep rift valley

– Widely scattered volcanoes

– Arctic and southwest India


Types of Spreading Centers


Convergent Boundary Features

• Plates move toward each other

• Oceanic crust destroyed

– Ocean trench

– Volcanic arc

• Deep focus earthquakes

– Great forces involved

– Mineral structure changes associated


Three Types of Convergent Boundaries


Types of Convergent Boundaries

• Oceanic-Continental

Convergence

– Ocean plate is

subducted

– Continental arcs

generated

– Explosive andesitic

volcanic eruptions



• Oceanic-Oceanic Convergence

– Denser plate is subducted

– Deep trenches generated

– Volcanic island arcs generated



• Continental-

Continental

Convergence

– No subduction

– Tall mountains uplifted

• Himalayas from India-

Asia collision


Transform Boundary Features

• Offsets oriented perpendicular to

mid-ocean ridge

– Segments of plates slide past

each other

• Offsets permit mid-ocean ridge to move apart

at different rates

• Shallow but strong earthquakes


Transform Boundary Features

• Oceanic Transform

Fault – ocean floor

only

• Continental

Transform Fault –

cuts across continent

– San Andreas Fault

• Transform faults

occur between

mid-ocean ridge

segments.


Applications of Plate Tectonics

• Mantle Plumes and

Hotspots

– Intraplate features

• Volcanic islands within

a plate

• Island chains

• Record ancient plate

motions

– Nematath – hotspot

track


Global Hotspot Locations


Hawaiian Island – Emperor Seamount

Nematath


Plate Tectonics and Intraplate Features

• Seamounts – Rounded tops

• Tablemounts or guyots– Flattened tops

• Subsidence of flanks of mid-ocean ridge

• Wave erosion may flatten seamount.


Coral Reef Development

• Fringing reefs –

develop along margin

of landmass

• Barrier reefs –

separated from

landmass by lagoon

• Atolls – reefs continue

to grow after

volcanoes are

submerged


Great Barrier Reef Records Plate Movement


Detecting Plate Motion with Satellites


Paleogeography

• Paleogeography – study of ancient continents

• Continental accretion

–Continental material added to edges of

continents through plate motion

• Pangaea – 540 million to 300 million years

ago


Breakup of Pangaea

• 180 million years ago – Pangaea separated

– N. and S. America rifted from Europe and

Africa

– Atlantic Ocean forms

• 120 million years ago – S. America and

Africa clearly separated

• 45 million years ago – India starts

Asia collision

– Australia moving north from Antarctica


Future Predictions

• Assume same direction and rate of plate

motions as now

– Atlantic will enlarge, Pacific will shrink

– New sea from East Africa rift valleys

– Further Himalaya uplift

– Separation of North and South America

– Part of California in Alaska


World Map 50 million Years in Future


Wilson Cycle

• John Tuzo Wilson

• Plate tectonics model shows life cycle of

ocean basins

– Formation

– Growth

– Destruction


Wilson Cycle


End of CHAPTER 2

Plate Tectonics and the Ocean Floor

chapter 1 clickers chapter 2 lecture essentials of ... · © 2014 pearson education, inc. plate...

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